The amino-oligopeptidases of rat small intestinal brush border are known to play a crucial role in assimilation of dietary protein. They will be thoroughly characterized structurally and their capacity to digest dipeptides with bulky side-chains will be defined. Regulation of the enzyme will be studied by use of monospecific antiserum from rabbits to isolate enzyme after pulse-labeling with radioactive amino acids and by identification of any precursor fragments reacting with the antiserum. The topography of these enzymes in the surface membrane will be determined by probing the luminal, intramembrane, and interior surface of the membrane with radioactive covalent ligands and isolating the aminopeptidases, their subunits and fragments. Finally a search will be made for intestinal amino-oligopeptidase deficiency in children with refractory diarrhea and malabsorption by assay of intestinal biopsy material and intestinal perfusion of peptides known to require this enzyme for their assimilation by the intestine.